Active and passive immunity to transforming proteins specified by sarc and leuk oncogenes provided highly significant protection against spontaneous and chemically induced cancers in mice and lethal transplantable tumors in rats as follows: 1) Spontaneous leukemias were essentially eliminated in AKR mice with the use of high titered passive antibodies (IgG). 2) Methylcholanthrene-induced sarcomas in C3H/f mice also were significantly prevented with the same passive anti-RadLV IgG. 3) Leukemias occurring late in life (550-650 days) in F1 offspring of C57L crossbreds were prevented by immunization of pregnant mothers with formalin inactivated GLV vaccine followed by immunization of Fl offspring with live MSV(GLV) vaccine. Transplants of syngeneic rat tumors of differing cell types induced by carcinogenic chemicals and by DNA or RNA tumor viruses were significantly prevented from replicating and killing F344 recipient rats when the rats were previously immunized with KiMSV rat tumor cell vaccines. Allogeneic and syngeneic KiMSV tumor cell vaccines provided immunosuppression of otherwise lethal syngeneic challenge tumors. The data showed that cancer could be prevented in mice and rats by active and transplantation immunity in each case specified by sarc and leuk transforming proteins of leuk and sarc oncogenes.